You can use thresholds to limit the number of times the system logs and displays intrusion events. This chapter covers the following sections:
Limiting Intrusion Event Logging
Thresholds, which you configure as part of your intrusion policy, cause the system to generate events based on how many times
traffic matching a rule originates from or is targeted to a specific address or address range within a specified time period.
This can prevent you from being overwhelmed with a large number of events. This feature requires a Protection license.
You can set event notification thresholds in two ways:
You can set thresholds per shared object rule, standard text rule, or preprocessor rule in your intrusion policy configuration,
as described in Configuring Event Thresholding.
By default, every intrusion policy contains a global rule threshold. The default threshold limits event generation for each
rule to one event every 60 seconds on traffic going to the same destination. This global threshold applies by default to all
intrusion rules and preprocessor rules. Note that you can disable the threshold in the Advanced Settings page in an intrusion
You can also override this threshold by setting individual thresholds on specific rules. For example, you might set a global
limit threshold of five events every 60 seconds, but then set a specific threshold of ten events for every 60 seconds for
SID 1315. All other rules generate no more than five events in each 60-second period, but the system generates up to ten events
for each 60-second period for SID 1315.
The following diagram shows an example where an attack is in progress for a specific rule. A global limit threshold limits
event generation for each rule to two events every 20 seconds.
Note that the period starts at one second and ends at 21 seconds. After the period ends, note that the cycle starts again
and the next two rule matches generate events, then the system does not generate any more events during that period.
Understanding Thresholding Options
Thresholding allows you to limit intrusion event generation by generating only a specific number of events in a time period,
or by generating one event for a set of events. When you configure global thresholding, you must first specify the thresholding
type, as described in the following table.
Table 1. Thresholding Options
Logs and displays events for the specified number of packets (specified by the count argument) that trigger the rule during
the specified time period. For example, if you set the type to Limit, the Count to 10 , and the Seconds to 60 , and 14 packets trigger the rule, the system stops logging events for the rule after displaying the first 10 that
occur within the same minute.
Logs and displays a single event when the specified number of packets (specified by the count argument) trigger the rule
during the specified time period. Note that the counter for the time restarts after you hit the threshold count of events
and the system logs that event. For example, you set the type to Threshold, Count to 10 , and Seconds to 60 , and the rule triggers 10 times by second 33. The system generates one event, then resets the Seconds and Count counters
to 0 . The rule then triggers another 10 times in the next 25 seconds. Because the counters reset to 0 at second 33, the system
logs another event.
Logs and displays an event once per specified time period, after the specified number (count) of packets trigger the rule.
For example, if you set the type to Both, Count to 2 , and Seconds to 10 , the following event counts result:
If the rule is triggered once in 10 seconds, the system does not generate any events (the threshold is not met)
If the rule is triggered twice in 10 seconds, the system generates one event (the threshold is met when the rule triggers
the second time)
If the rule is triggered four times in 10 seconds, the system generates one event (the threshold is met when the rule triggered
the second time and following events are ignored)
Next, specify the tracking, which determines whether the event instance count is calculated per source or destination IP address.
Finally, specify the number of instances and time period that define the threshold.
Table 2. Thresholding Instance/Time Options
The number of event instances per specified time period per tracking IP address or address range required to meet the threshold.
The number of seconds that elapse before the count resets. If you set the threshold type to Limit, the tracking to Source, Count to 10, and Seconds to 10, the system logs and displays the first 10 events that occur in 10 seconds from a given source port. If only seven
events occur in the first 10 seconds, the system logs and displays those, if 40 events occur in the first 10 seconds, the
system logs and displays 10, then begins counting again when the 10-second time period elapses.
Configuring Global Thresholds
You can set a global threshold to manage the number of events generated by each rule over a period of time. When you set
a global threshold, that threshold applies for each rule that does not have an overriding specific threshold. For more information
on configuring thresholds, see Understanding Thresholding.
A global threshold is configured on your system by default. The default values are as follows:
From the Type radio buttons, select the type of threshold that will apply over the time specified by the seconds argument. See the Table 1 table for more information:
Select Limit to log and display an event for each packet that triggers the rule until the limit specified by the count argument is exceeded.
Select Threshold to log and display a single event for each packet that triggers the rule and represents either the instance that matches
the threshold set by the count argument or is a multiple of the threshold.
Select Both to log and display a single event after the number of packets specified by the count argument trigger the rule.
Select the tracking method from the Track By radio buttons:
Select Source to identify rule matches in traffic coming from a particular source IP address or addresses.
Select Destination to identify rule matches in traffic going to a particular destination IP address.
In the Count field:
For a Limit threshold, specify the number of event instances per specified time period per tracking IP address required to meet the threshold.
For a Threshold threshold, specify the number of rule matches you want to use as your threshold.
In the Seconds field:
For a Limit threshold, specify the number of seconds that make up the time period when attacks are tracked.
For a Threshold threshold, specify the number of seconds that elapse before the count resets. Note that the count resets if the number of
rule matches indicated by the Count field occur before the number of seconds indicated elapse.
By default, a global limit threshold limits the number of events on traffic going to a destination to one event per 60 seconds.
You can disable global thresholding in the highest policy layer if you want to threshold events for specific rules and not
apply thresholding to every rule by default.